Etching plate and method



United States Patent 3,125,788 ETCHING PLATE AND METHOD Erich Pelzel, Stolberg, Rhineland, Germany, assignor to Stolberger Zink Aktiengesellschaft fuer Bergbau und Huettenbetriebe, Aachen, Germany No Drawing. Filed July 16, 1959, Ser. No. 827,425 Claims priority, application Germany Jan. 24, 1959 7 Claims. (Cl. 22-212) This invention relates to a novel etching plate and a method for its production. More particularly, the present invention is concerned with etching plates made from certain zinc alloys by a specific crystallization process.

It is already known to produce etching plates from zinc containing up to 0.4% of lead, 0.2% of cadmium, 0.01% of iron and in some cases also such alloying metals as up to 0.035% of magnesium and aluminum by pouring the molten metal into flat open molds, processing the ingot thus obtained so as to remove oxidized metal, and rolling the ingot thus processed into a sheet of metal suitable for use as etching plate.

When using these prior art etching plates, it is common practice to work on the side corresponding to the bottom of the zinc ingot, assuming that the side of the ingot where the primary crystallization takes place consists of a mass of particularly fine crystals, which are best suited for etching purposes. For this reason, only relatively thin ingots, for example ingots having a thickness of 16 mm., have heretofore been used in the production of etching plates.

However, experience has shown that the etching plates produced in this manner are not entirely satisfactory. Thus, in deep etching the flat flanks required by the etcher cannot always be obtained, although in surface etching the structureless picture of the surface is generally adequate. Instead of the desired flat flanks, strong underetchings, making the plate unsuitable for use, are frequently obtained in deep etching when working with the zinc etching plates of the prior art. In those cases where the requirement of flat flanks can be met, the lines of furrows obtained are usually not clean and contain so-called pimples. Moreover, the! etching velocity obtainable with these plates is low and the etching time is many times as long as the usual etching time. Consequently, these etching plates, too, were unsuitable for practical use.

It is an object of the present invention to provide an etching plate based on zinc which avoids the disadvantages of the prior art zinc plates.

Another object of the present invention is to provide an etching plate based on zinc which meets all requirements of the etching craft.

A further object of the present invention is to provide an etching plate which gives a structureless picture in surface etching and particularly fiat flanks in deep etching.

Still another object of the present invention is to provide an etching plate based on zinc which is distinguished by a high etching velocity.

A further object of the present invention is to provide an etching plate based on zinc which gives clean lines and furrows without pimples consisting of undissolved coarse crystals.

Still another object of the present invention is to provide a novel process for making etching plates.

Still further objects will appear hereinafter.

With the above objects in view, the present invention provides an etching plate made by casting a molten zinc alloy in such manner as to effect ingot segregation into two phases, one of said phases being rich in primary zinc crystals formed from the portion of the melt first solidified and the other of said phases being rich in finely grained crystals formed from the residual melt; and processing the casting to thereby obtain a sheet of zinc metal,

3,125,788 Patented Mar. 24, 1964 said sheet being particularly suitable for etching purposes on the surface formed from the residual melt.

The present invention is based on the discovery that certain alloys of zinc when molten and poured into molds lying on the ground and being cooled at the bottom, solidify at first at the bottom with formation of primary crystals of the base metal, i.e. zinc, whereas the metals alloyed with the zinc remain in the residual melt. As a result, the ingot obtained comprises two phases, a phase rich in primary zinc crystals and a phase rich in mixed crystals formed from the residual melt. This phenomenon is commonly referred to as ingot segregation.

When making a study of the zinc alloys which are described below, it was found that in the process of ingot segregation, the residual melt lying before the upwardly moving solidification front spontaneously forms crystal seeds at a certain moment and then solidifies with the formation of fine grains. The fracture of a zinc ingot made in accordance with the present invention therefore shows dendritic crystallites in the rapidly cooled bottom portion, said crystallites standing vertically to the plane of the mold, and an unoriented, finely grained texture in the upper portion. Crystallographically, the bottom portion of such an ingot consists of a texture oriented to one side while the top portion is made up of entirely unoriented fine crystals.

If an ingot of such structure is rolled out into a metal sheet of, say 2 mm. thickness the surprising observation is made that the surface of the sheet which corresponds to the bottom side of the ingot and consists of the dendritic oriented crystals is not as suitable for etching purposes as the surface of the sheet corresponding to the top of the ingot and consisting of fine unoriented crystals. When using the latter surface of the sheet in surface etching good, structureless pictures are obtained, while deep etching on this surface is distinguished by particularly flat flanks, high etching velocity, and clean lines free from pimples due to undissolved coarse crystals.

It has also been found that in making the etching plates of this invention thicker ingots can be cast. This constitutes a further advantage since thicker ingots permit a higher yield of sheet metal, the quantity of reject being smaller. Moreover, the manufacture of the etching plates is more economical in view of the savings in labor made possible and the better utilization of the rolling mills.

Zinc alloys which are suitable for use in the practice of the present invention and which when molten and subjected to proper crystallization conditions will give ingot segregation include the alloys of zinc with small amounts of lithium, cadmium, aluminum, magnesium, beryllium, titanium, zirconium, hafnium and other metals capable of forming mixed crystals with zinc. Such alloys can be obtained in known manner by adding to pure zinc small and critical a-mounts of one or more of these metals, which are apt to cocrystallize 'with zinc. The most advantageous concentrations of the metals used to make the preferred zinc alloys like below 1% are listed in Table I.

TABLE I Concentration Concentration Name of Metal in Original in Residual Melt (in Melt (in weight percent) weight percent) Aluminum 0. 003-0. 2 0. 005-0. 3 Magnesium 0. 003-0. 2 0. 005-0. 3 Lithium 0. 001-0. 05 0. 0015-0. 075 CadmiuJJL 0. 001-0. 5 0. 0015-0. gerymum 0 0015-0 0075 itam'um Zirconium 0. 001-0. 005 Hafnium Particularly preferred are alloys of zinc with aluminum and/or magnesium having the compositions indicated in Table 1. However, aluminum and/or magnesium may also be present in higher concentrations although it is advisable to keep the aluminum and/or magnesium content of the alloy below 1% of the zinc. If both magnesium and aluminum are present, the ratio of magnesium to aluminum is advantageously within the range of 1:1 to 1:3 and preferably about 1:2. On the other hand, if beryllium, titanium, zirconium and/ or hafnium are used concurrently with aluminum and/ or magnesium, their total amount should not exceed half of the aluminum and/ or magnesium content of the zinc alloy.

Using a zinc alloy of the above-described composition ingot segregation can be effected in any suitable manner. A preferred way of bringing about ingot segregation according to this invention involves pouring a melt of the zinc alloy into a mold; allowing the melt to cool down to a temperature somewhat above the melting point of zinc; cooling the lower portion of the melt so as to cause part of the zinc to solidify in the form of dendritic crystals, and heating the residual melt, enriched in the metal constituent(s) of the alloy capable of forming mixed crystals with zinc, so as to reduce the cooling velocity of the residual melt and to slow down its solidification, thereby causing the residual melt to form very fine unoriented crystals.

In this crystallization process, which constitutes the preferred way of bringing about ingot segregation according to this invention, the initial temperature of the melt must be above the melting point of zinc in order to avoid a premature solidification. For the purpose of insuring a sufficiently high initial temperature, the mold may be heated. However, it is frequently more convenient to pour an overheated melt into a mold of ambient temperature.

The temperature T in C. to which a zinc melt (melting point of zinc: 420 C.) has to be heated when using a mold of ambient temperature can be calculated by means of the following equation 'w s (420 7.0152 in which W is the weight of the mold, s is the specific heat of the material of construction of the mold, W2 is the weight of the zinc melt, s is the specific heat of the zinc melt and t is the temperature in C. of the mold.

To cite an example, if '80 kg. of a zinc melt having a specific heat of 0.1 are subjected to ingot segregation according to this invention in an iron mold weighing 20 kg., having a specific heat of 0.13 and having a temperature of 20 C. before the zinc melt is poured into the mold, T is given by Hence,

In the manner described above, ingots of almost any desired thickness can be cast. In general, it is advantageous to make ingots having a thickness of 50 mm., 60 mm., 100 mm., or more.

Since the finely crystalline portion of the ingot gives the etching surface while the dendritic portion is less suited for etching purposes, it is advisable to cast ingots, the dendritic portion of which is as small as possible. For plates used in deep etching, the finely crystalline proportion should have a depth of at least 0.3 mm. and preferably 1.0 mm. This means that an ingot containing at least 15% of finely crystalline material is required to make a satisfactory sheet of 2 mm. thickness.

As may be seen with the naked eye when inspecting a fracture of the etching plates of this invention, they are distinguished by a texture consisting of fine unoriented crystals but also by the fact that each individual crystal 4': contained therein is considerably smaller than the crystallites present in the known etching plates.

According to a preferred embodiment of the present invention, the above-described crystallization process resulting in ingot segregation is carried out in such manner as to obtain fine crystals of a grain size, expressed in terms of grain area after grain area etching, of less than 0.2 mm. preferably of 0.1 mm. or less.

The determination of the grain area after grain area etching can be carried out by the method of R. Dederichs and H. Kostron published in Archiv fiir Metallkunde, volume 3 (1949), pages 193-199.

The etching plates of the present invention may be made from the segregated ingots by any suitable metal processing operation. Thus, the casting may be processed in a rolling-mill where the work is performed by passing the casting through one pair of heavy rollers after another until a sheet of, say, 2 mm. thickness is obtained. When using the etching plate thus produced, the side of the sheet corresponding to the upper side of the ingot serves as the etching surface.

Another method of making the etching plates of this invention from the segregated ingots involves removing the lower portion of the casting, which consists of the coarse dendritic crystals, by cutting, planing or shearing it off. In this manner, the lower side of the finely crystalline portion is laid bare. After rolling out the finely crystalline ingot section, one obtains a sheet, both surfaces of which are highly suitable for etching.

The following example is additionally illustrative of the present invention but is not to be construed as limiting the scope thereof.

Example A zinc melt made up of pure zinc, 0.1% of aluminum and 0.05% of magnesium and heated to a temperature of about 450 C. is poured into a flat, open mold made of iron sheet of 5 mm. thickness and heated to a temperature of 430 C. By using a mold heated to this temperature, a premature solidification of the melt is avoided. Mold and melt are then allowed to cool down to a temperature slightly above the melting point of zinc (420 C.). Now the lower part of the mold is cooled vigorously to thereby bring about solidification, in the form of dendritic crystals, of the lower portion of the melt. The mass of dendritic crystals thus formed is oriented vertically to the plane of solidification. As the crystallization progresses the residual melt is being enriched in magnesium and aluminum. When a certain concentration of these two metals is reached, the number of seeds increases spontaneously. As a result, the residual melt begins to crystallize. During this stage, the surface of the mass is heated to slow down the solidification of the residual melt. In this manner, exceedingly fine unoriented crystals are obtained from the residual melt.

The ingot thus produced, having a finely crystalline texture in its upper part and a dendritic texture in its lower part, is rolled out into a sheet of 2 mm. thickness. When using this sheet as an etching plate the etching is done on the surface corresponding to the top of the ingot.

While the invention has been described with reference to specific embodiments, it will be apparent to those skilled in the art that various modifications may be made and equivalents substituted therefor without departing from the principles and true nature of the present invention.

What is claimed by Letters Patent is:

1. A process for producing an etched plate which comprises pouring a melt made up of pure zinc, aluminum and magnesium into a shallow mold, the total amount of magnesium and aluminum being between 0.003% and less than 1% of the zinc and the ratio of magnesium to aluminum being within the range of about 1:1 to 1:3; allowing the melt to cool down to a temperature somewhat above the melting point of zinc; cooling the lower portion of the melt so as to cause ingot segregation, part of the zinc solidifying in the lower portion of said shallow mold in the form of a layer of coarse, dendritic crystals; heating the residual melt located above said dendritic layer so as to slow down its solidification, thereby causing the residual melt to form a layer of unoriented fine crystals coextensive with and located above said dendritic layer; processing the ingot thus made having a finely crystalline upper section and a dendritic lower section so as to form a plate adapted to be etched and having two opposite surface portions, one of said opposite surface portions being composed of said fine unoriented crystals and the other of said surface portions having a dendritic structure; and etching said one of said opposite surface portions.

2. A process as defined in claim 1 wherein said ingot has a thickness of at least 50 mm.

3. A process as defined in claim 1 wherein a finely crystalline upper section constituting at least 15% of said ingot is formed.

4. A process for producing an etched plate which comprises pouring a melt made up of pure zinc, aluminum and magnesium into a flat mold, the total amount of magnesium and aluminum being between 0.003% and less than 1% of the zinc and the ratio of magnesium to aluminum being within the range of about 1:1 to 1:3, allowing the melt to cool down to a temperature some what above the melting point of zinc; cooling the lower portion of the melt so as to cause ingot segregation, part of the zinc solidifying in the lower portion of said flat mold in the form of a layer of coarse, dendritic crystals; heating the residual melt located above said dendritic layer so as to slow down its solidification, thereby causing the residual melt to form a layer of unoriented fine crystals coextensive with and located above said dendritic layer; rolling the ingot thus made into a plate adapted to be etched, said plate having two opposite surface portions, one of said opposite surface portions being composed of said fine oriented crystals and the other of said surface portions having a dendritic structure; and etching said one of said opposite surface portions.

5. A process for producing a plate adapted to be etched, which comprises pouring a melt made up of pure zinc, aluminum and magnesium into a shallow mold, the total amount of magnesium and aluminum being between 0.001% and 0.5% of the zinc and the ratio of magnesium to aluminum being within the range of about 1:1 to about 1:3; allowing the melt to cool down to a temperature somewhat above the melting point of zinc; cooling the lower portion of the melt so as to cause ingot segregation, part of the zinc solidifying in the lower portion of said shallow mold in the form of a layer of coarse, dendritic crystals; heating the residual melt located above said dendritic layer so as to slow down its solidification, thereby causing the residual melt to form a layer of unoriented fine crystals coextensive with and located above said dendritic layer; separating the ingot thus made into a lower section including said dendritic crystals and an upper section including said fine crystals; and rolling the upper section into a plate adapted to be etched on both surfaces.

6. A process for producing an etched plate which comprises pouring a melt made up of pure zinc, aluminum and magnesium into a shallow mold, the total amount of magnesium and aluminum being between 0.003% and 0.2% of the zinc and the ratio of magnesium to aluminum being within the range of about 1:1 to about 1:3; allowing the melt to cool down to a temperature somewhat above the melting point of zinc; cooling the lower portion of the melt so as to cause ingot segregation, part of the zinc solidifying in the lower portion of said shallow mold in the form of a layer of coarse, dendritic crystals; heating the residual melt located above said dendritic layer so as to slow down its solidification, thereby causing the residual melt to form a layer of unoriented fine crystals coextensive with and located above said dendritic layer; rolling the ingot thus made into a plate adapted to be etched and having two opposite surface portions, one of said opposite surface portions being composed of said fine unoriented crystals and the other of said surface portions having a dendritic structure; and etching said one of said opposite surface portions.

7. A process for producing a plate adapted to be etched, which comprises pouring a melt made up of pure zinc, aluminum and magnesium into a shallow mold, the total amount of magnesium and aluminum being between 0.003% and 0.2% of the zinc and the ratio of magnesium to aluminum being within the range of about 1:1 to about 1:3; allowing the melt to cool down to a temperature somewhat above the melting point of zinc; cooling the lower portion of the melt so as to cause ingot segregation, part of the zinc solidifying in the lower portion of said shallow mold in the form of a layer of coarse, dendritic crystals; heating the residual melt located above said dendritic layer so as to slow down its solidification, thereby causing the residual melt to form a layer of unoriented fine crystals coextensive with and located above said dendritic layer; separating the ingot thus made into a lower section including said layer of dendritic crystals and an upper section including said layer of fine crystals; and rolling the upper section into a plate adapted to be etched on both surfaces.

References Cited in the file of this patent UNITED STATES PATENTS 1,777,659 Stay et a1. Oct. 7, 1930 2,060,919 Anstey et al Nov. 17, 1936 2,141,813 Finkelday Dec. 27, 1938 OTHER REFERENCES Sayre, TR 940, S 37, 1951, C. 2, 5th ed., 1951, C1. 96 (Photography and Platemaking for Photo-Lithography, pp. 31, 32 and 33 relied upon). 

1. A PROCESS FOR PRODUCING AN ETCHED PLATE WHICH COMPRISES POURING A MELT MADE UP OF PURE ZINC, ALUMINUM AND MAGNESIUM INTO A SHALLOW MOLD, THE TOTAL AMOUNT OF MEGNESIUM AND ALUMINUM BEING BETWEEN 0.003% AND LESS THAN 1% OF THE ZINC AND THE RATIO OF MAGNESIUM TO ALUMINUM BEING WITHIN THE RANGE OF ABOUT 1:1 TO 1:3; ALLOWING THE MELT TO COOL DOWN TO A TEMPERTURE SOMEWHAT ABOVE THE MELTING POINT OF ZINC; COOLING THE LOWER PORTION OF THE MELT SO AS TO CAUSE NGOT SEGREGATION, PART OF THE ZINC SOLIDIFYING IN THE LOWER PORTION OF SAID SHALLOW MOLD IN THE FORM OF A LAYER OF COARSE,DENDRITIC CRYSTALS; HEATING THE RESIDUAL MELT LOCATED ABOVE SAID DENDRITIC LAYER SO AS TO SLOW DOWN IN SOLIDIFICATION, THEREBY CAUSING THE RESIDUAL MELT TO FORM A LAYER OF UNORIENTED FINE CRYSTALS COEXTENSIVE WITH AND LOCATED ABOVE SAID DENDRITIC LAYER; PROCESSING THE INGOT THUS MADE HAVING A FINELY CRYSTALINE UPPER SECTION AND A DENDRITIC LOWER SECTION SO AS TO FORM A PLATE ADAPTED TO BE ETCHED AND HAVING TWO OPPOSITE SURFACE PORTIONS, ONE OF SAID OPPOSITE SURFACE PORTIONS BEING COMPOSED OF SAID FINE UNORIENTED CRYSTALS AND THE OTHER OF SAID SURFACE PORTIONS HAVING A DENDRITIC STRUCTURE; AND ETCHING SAID ONE OF SAID OPPOSITE SURFACE PORTIONS. 